This invention relates to turbocharger systems and more particularly relates to multiple turbocharger systems.
Internal combustion engines are frequently equipped with turbo-superchargers to provide a boost in horsepower. However, the boost and usefulness of the turbocharger is usually limited to a very narrow range of engine speeds. The turbine section of the turbo-supercharger essentially depends upon exhaust gas volume and exhaust gas pressure to drive the centrifugal compressor portion, more commonly referred to as the supercharger.
Conventional turbochargers have a very limited operational efficiency range that limits their maximum effectiveness to approximately 25% of the range of an engine's RPM capability. This means that it is possible to select a turbine size that will function well within a narrow band of an engine's total RPM range. Thus, a small turbine will function well at low engine speeds (RPM); whereas, a large one will not function at all at these speeds. Conversely, a large turbine will function well at high engine speeds (RPM), while a small turbine, though able to function with a low volume of exhaust gases provided by an engine at low speeds, will produce excessive back pressure at high engine RPM. In fact a small turbine can actually be detrimental to engine performance at higher RPM, so much so that the engine at times can perform better in that range without the turbocharger. The large turbine, though it would not produce high back pressure at high engine speed, is conditionally inefficient because it simply will operate very little or not at all with the low volume of exhaust gas available at low engine speeds.
As the air flow requirements of a turbo supercharged engine increase, more turbine power must be provided in order to rotate the centrifugal compressor to meet the increased flow requirements. One traditional method of accomplishing this is to allow the exhaust gas pressure that is entering the turbine to increase proportionately. Consequently, numerous turbocharger arrangements have been devised to alleviate the problems. Examples of devices used to alleviate this problem are systems with two completely separate turbochargers or the use of a dual scroll with a single turbine wheel.
However, these devices necessarily result in complex configurations requiring sophisticated control systems and have not been particularly effective in specialized vehicle applications. The disadvantage of the present dual scroll turbines used in conjunction with dual exhaust systems to increase efficiency is that although they have separate inlets, they direct the exhaust gas flow through a common annular space to react against a single turbine wheel. This allows normally isolated engine exhaust gas streams to partially react against each other, thereby decreasing engine breathing efficiency. This defeats the purpose of isolated exhaust manifolds to prevent interference at overlap events. (An overlap event occurs when both intake and exhaust valves are opened simultaneously in the same cylinder.) Exhaust gas that initially leaves the cylinder when an exhaust valve opens is at high pressure and flows at high velocity. The flow that exists when an exhaust valve is closing and intake is opening referred to as an overlap event is at low pressure and low velocity. With some of the prior art systems, high-pressure events are not isolated from low-pressure events, allowing exhaust gas reversion.